Not Your Grandfather's Five Senses

Your 21 (or more) Senses, From The New Scientist.

While schools still teach us that there are five senses - an idea that came courtesy of Aristotle and permeates popular culture - the count is at odds with science. Try grabbing an ice cube with one hand and a red-hot poker with the other, and tell me that what you feel can be encompassed by the favourite five. Go on a white-knuckle ride at any theme park and convince me that everything you experienced was down to sight, sound and touch. You probably had your eyes closed anyway. There is clearly more to sensation than these five categories. So how many senses do we have?

In some ways the answer depends on how we divide our sensory systems up. For example, we could classify senses by the nature of the stimulus. In this sense (as it were) there are just three types, not five - chemical (sensed as tastes, smells or “internally”, as with blood glucose), mechanical (touch and hearing) and light (vision). Some animals also have electroreception or a magnetic sense. All these groups of sensation require quite different sensory systems. Something dissolving on the tongue and producing an odour which permeates up into the nose and fits into a receptor is quite different from the mechanical movement of a hair cell in the inner ear, or a photon hitting the retina.

The Five Senses

But we could as easily subdivide these further, and define a “sense” as a system consisting of a specialised cell type responding to a specific signal and reporting to a particular part of the brain. For instance, taste could be seen not as one sense but five - sweet, salt, sour, bitter and “umami”, a Japanese word for the taste of glutamate, which gives us our sense of meaty flavours. Vision could be viewed as one sense (light), two (light and colour) or four (light, red, green and blue). In some animals there are retinal cells which respond only to movement. Some people might consider that to be yet another sense. Neurologists classify pain as cutaneous, somatic or visceral depending on where it is felt - but does this mean they are different sensory systems or are they simply a matter of geography on and in the body?

Many people would agree that they can sense temperature, pressure, touch, joint position (proprioception), body movement (kinaesthesis), balance and feelings associated with a full bladder, an empty stomach or thirst. But there are other monitoring systems in the body that we can never be even dimly “aware of” - sensing the pH of the cerebrospinal fluid would be an example.

And take hearing. Is this one sense, or many hundreds, one per cochlear hair cell? That is probably taking things a bit too far, but it is interesting to note that we can lose high-frequency hearing without losing low-frequency acuity, and vice versa. So maybe they should be thought of separately. The more we study the structure of our sense organs, the more senses we appear to have.

But, intriguing as all this is, sensation alone isn’t really all that important. When we talk of senses, what we really mean are feelings or perceptions. Otherwise we’d be operating not much above the level of an amoeba or a plant. The majority of the natural world gets by with just one or two senses - typically light and touch. A plant that grows to follow the apparent motion of the sun or the Venus fly-trap closing over an insect is merely reacting mechanically to a stimulus.

We, on the other hand, see light and shade but perceive objects, spaces and people, and their positions. We hear sounds, but we perceive voices or music or approaching traffic. We taste and smell a complex mixture of chemical signals, but we perceive the mix as ice cream or an orange or a steak. Perception is the “added value” that the organised brain gives to raw sensory data. Perception goes way beyond the palette of sensations and involves memory, early experiences and higher-level processing.

What you hear, for example, is not just a simple sum of the sounds collected by each ear, but a bigger picture. Various processes come into play, some of which allow the brain to tell the direction of the noise. Even more complex processes enable us to screen out one sound when attending to another. In the well-known “cocktail party phenomenon”, for example, we ignore all extraneous sounds while taking part in a conversation, but can quickly switch focus if someone else mentions our name. The implication is that we were always “listening” to ambient sound but not always “hearing” it, except when it suddenly becomes meaningful. Our perception goes far beyond the bare sensation.